Problem: The scale on a map is 9cm : 6km. If two cities are 18cm apart on the map, what is the actual distance in km between the two cities?
Solution: The scale means that every 9cm on the map corresponds to an actual distance of 6km. A distance of 18cm on the map is the same as 2 $\cdot$ 9cm. The actual distance between the two cities is 2 $\cdot$ 6km, or 12km.